It seems to me that Nazgul cannot see visible, or "White", light. If they could, they would see the living as more than just shadows. You see, cameras designed specifically to pick up UV light and nothing else will almost always create "black and white" photos, or "shadowy" looking objects. This being the case, I could believe that Nazgul see in UV, but not in Infrared. The wavelengths are far too different and on opposite sides of the visible spectrum. It wouldn't make sense that they could pick up light of such widely different wavelengths, but not the wavelengths in between.
As for Elves using UV to see which snow can be stepped on, I'm afraid not. You see, snow relfects UV light so well that no difference can be seen between varying depths, etc, as you can see
here. The picture on the left is with a regular camera, and the one on the right is taken with a UV camera. As you can see there are no differences in the snow. Also,
this picture was taken under UV, and there is no difference in the snow.
However, polarized white light is plausible.
Here is an article about NASA using the same method to prepare snow for the different events in the winter Olympics. Using polarized white light, they can tell how hard or soft the snow or ice is, and even how dense it is.
Though it is still possible that Elves could view UV light. Many birds do this. Here's a quote from an article on it:
Quote:
Birds, though, see differently than we. They can see in the UV, which we cannot. Birds seem to use their UV vision much as the rest — finding prey and mates.
Also, like many reptiles, birds see four primary colors (that is, they have four types of color cones in their eyes that receive light), not merely three like bees or us . Their 4-color system may produce "a range of hues we cannot imagine," says Innes Cuthill, professor of behavioral ecology at the University of Bristol
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So, it is entirely possible that Elves have a 4th type of cone in their eye, and infact see in 4 primary colors. Could explain the difference in their art.